1. Field of the Invention
The present invention relates to a WDM (Wavelength Division Multiplexing)-PON (Passive Optical Network) system, and more particularly to a WDM light source apparatus.
2. Description of the Related Art
Typically, a WDM-PON system provides subscribers with super high-speed broadband communication service using unique wavelengths assigned to each subscriber. Using such a configuration, WDM-PON systems can (1) guarantee communication confidentiality, (2) accommodate additional communication service requests from individual subscribers or increased communication capacity, and (3) increase the number of subscribers by further including a unique wavelength to be assigned to a new subscriber.
However, such WDM-PON systems must use the selected light source of a specified wavelength and an additional wavelength stabilizer to stabilize the light source of a specified lasing wavelength at a CO (Central Office) and individual subscriber ends This results in a high cost of use that is assessed to the subscriber. Because of these financial costs, such WDM-PON systems are not commercially available.
Several conventional light sources for WDM systems are known. These include a DFB (Distributed FeedBack) laser array, a MFL (Multi-Frequency Laser), a spectrum-sliced light source, and a mode-locked Fabry-Perot laser with incoherent light, etc. as WDM light sources.
The DFB laser array and the MFL are manufactured in a complicated process, and are high-priced elements that must select a correct wavelength and stabilize a wavelength so as to implement a WDM scheme.
The mode-locked Fabry-Perot laser with incoherent light performs spectrum-slicing on a broadband optical signal created from an incoherent light source such as an LED and an optical fiber amplifier light source, etc. using an optical filter or a WGR (Waveguide Grating Router). The spectrum-sliced broadband optical signal is injected into a Fabry-Perot laser having no isolator. The resultant mode-locked signal is then adapted to a signal transmission mode. If a spectrum-sliced signal of more than a predetermined output level is applied to the Fabry-Perot laser, the Fabry-Perot laser generates only a wavelength equal to that of the received spectrum-sliced signal.
The mode-locked Fabry-Perot (FP) laser with incoherent light directly modulates a low-priced Fabry-Perot laser signal according to a data signal, and is suitable for high-speed long distance transmission. However, in order to allow the Fabry-Perot laser to output a mode-locked signal suitable for high-speed long distance transmission, a high-output incoherent light signal having a wide bandwidth must be applied to the Fabry-Perot laser device. This requires that the Fabry-Perot laser use an expensive high-output broadband light source. Furthermore, there arises a mode hopping phenomenon arises in such lasers. Temperature variations may cause an output signal mode of the mode-locked Fabry-Perot (FP) laser is to change to another mode. When this phenomenon happens, data being transferred at a high speed may be lost.
Another light source is the spectrum-sliced light source. Such light sources perform spectrum-slicing on a broadband optical signal using a WGR (Waveguide Grating Router). This provides users with a large number of WDM channels. Attempts have been made to use an LED (Light Emitting Diode), a SLD (SuperLuminescent Diode), a FP (Fabry-Perot) laser, a fiber amplifier light source, and an ultra-short pulse light source as such a spectrum-sliced light source.
While an LED and an ultra LED proposed as such a spectrum-sliced light source have an optical bandwidth and are low-priced elements, these elements have a low modulation bandwidth and a low output level. Therefore, they may be appropriate as a light source for use with an upstream signal having a modulation rate lower than that of a downstream signal.
Also, while the Fabry-Perot laser is a low-priced high output element, it has a narrow bandwidth, such that it is impossible to provide a large number of wavelength division channels. Also when modulating and transmitting a spectrum-sliced signal at a high speed, the Fabry-Perot may have performance degradation due to a mode partition noise.
It is also noted that while the ultra-short pulse light source adapts a wide spectrum bandwidth as a spectrum bandwidth of a light source and has coherent characteristics, it has a low stability of a lasing spectrum and a narrow pulse width of several picoseconds (ps). This results in practical implementation difficulties.
Yet another possible WDN light source is a spectrum-sliced fiber amplifier light source. Such a light source provides a user with a large number of wavelength-division high-output channels by performing spectrum-slicing on the ASE light created from the optical fiber amplifier. However, this spectrum-sliced light source must use a high-priced external modulator such as a LiNb03 to allow individual channels to transmit different data.
In conclusion, there is a need in the art for a cost-effective WDM light source apparatus applicable to WDM-PON systems.